1. Field of the Invention
This invention relates to an improved method and apparatus for installing a superstructure onto an offshore platform. More particularly, the invention relates to an improved method and apparatus for installing a deck structure onto an offshore platform in exposed water offshore using a barge mounted, cantilevered support structure.
2. Description of the Prior Art
Historically, the placement of a deck structure or a superstructure onto an offshore platform has been performed either by setting the deck or superstructure onto the platform using the crane of a derrick barge or lowering the deck onto the platform using a float-on, deck-setting procedure. This second method typically involves the maneuvering of the deck structure into an existing slot in the platform at the water line and then ballasting the barge which is temporarily supporting the deck until the deck structure engages the offshore platform. At that point, the deck loads have been transferred to the platform and the ballasted barge may be removed from the platform slot. A general discussion of the float-on procedure is shown in FIGS. 1 and 2 of U.S. Pat. No. 4,662,788 and the accompanying detailed description.
Both of these methods, however, have distinct disadvantages. The use of a crane on a derrick barge has lifting limitations primarily controlled by the lifting capacity of the crane, which is largely a function of the angle of the main boom, and the size of the derrick barge. Frequently, in designing the deck structure, the engineer must take into account the capacity of available derrick barges. The alternate float-on, deck-setting procedure also has significant limitations in that the offshore platform either must have a large enough water plane to provide enough of a slot between the principal support legs of the platform to accommodate a barge large enough to support the deck structure during the installation phase or the operation must be performed in a sheltered environment using multiple barges moored around the platform to create enough capacity to support a deck and permit a deck setting operation.
Therefore, the need exists for an improved method and apparatus for setting a deck structure or a superstructure which does not increase the expense associated with a large derrick barge but has the flexibility of accommodating smaller offshore platforms which do not have a large enough in-place water plane to support the float-on method.
U.S. Pat. No. 4,829,924 to Dysarz discloses a semisubmersible vessel 10 to set a deck structure 17 onto a fixed offshore platform 15. However, the procedure disclosed by Dysarz is complex and requires an elaborate semisubmersible to control the vertical displacement of a deck structure which is supported off one end of the barge. Furthermore, the disclosure by Dysarz still has substantial limitations associated with the float-on method in that the trusses 11 which support the lifting beams 9 are on the interior of the jacket 15 and would interfere with the lowering of the deck superstructure on smaller platforms for the same reason that the float-on method is inadequate in view of the smaller interior water planes. Dysarz also requires a complex curved guide frame 7 to assist the semisubmersible vessel 10 and lifting beams 9 during the raising and lowering phases.
Accordingly, there is a need for an improved method and apparatus for installing a deck structure or a superstructure which is inexpensive and simple and avoids the complications of the Dysarz disclosure.